Key CLARITY technologies I - Quantum Cascade Lasers National and Kapodistrian University of Athens Department of Informatics and Telecommunications Photonics Technology Laboratory Key CLARITY technologies I - Quantum Cascade Lasers

Introduction - Bipolar lasers In usual laser diodes, transitions occur between different electronic bands of the semiconductor crystal (inter-band transitions). A photon is emitted when an electron jumps from a semiconductor's conduction band (CB) to a hole in the valence band (VB). Once an electron has been neutralized by a hole it can emit no more photons. The wavelength of the photon is determined by the semiconductor bandgap and it is usually in the near infrared region. CB bandgap VB

Introduction - Intersubband lasers The Quantum Cascade Laser (QCL) is a semiconductor laser involving only one type of carriers. It is based on two fundamental quantum phenomena: - the quantum confinement - the tunneling In the QCL the laser transitions do not occur between different electronic bands (CB- VB) but on intersubband transitions of a semiconductor structure. An electron injected into the gain region undergoes a first transition between the upper two sublevels of a quantum well and a photon is emitted. Then the electron relaxes to the lowest sublevel by a non-radiative transition, before tunneling into the upper level of the next quantum well. The whole process is repeated over a large number of cascaded periods. CB

Introduction - Bipolar lasers vs QCLs Diode Laser Light from electron-hole (e-h) recombination Emission wavelength controlled by bandgap Wide gain spectrum due to broad thermal distribution of e, h One photon per injected e-h pair above threshold Gain limited by band-structure (absorption coefficient) Quantum Cascade Laser Light from quantum jumps between subbands Emission wavelength controlled by thickness: (4 to 160m) Narrow gain spectrum due to same curvature of the initial and final states No threshold for population inversion: gain form the first flowing electron. Gain limited by electron density in the excited state (i.e. by maximum current one can inject) and cavity losses Large gain: above threshold N photons per injected electron are generated (N: number of cascaded stages) bandgap CB VB layer thickness CB

Milestones 1971: First proposal for use of inter-subband transition (Ioffe Inst.) Kazarinov, R.F; Suris, R.A., "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice“, Soviet Physics - Semiconductors 5, 707–709, 1971. …. 1985: First observation of intersubband absorption in superlattice QW L. C. West and S. J. Eglash, “First observation of an extremely large‐dipole infrared transition within the conduction band of a GaAs quantum well”, Applied Physics Letters, 46, 1156-1158, 1985. 1986: First observation of sequential resonant tunneling in superlattice QW F. Capasso, K. Mohammed, and A. Y. Cho, “Sequential resonant tunneling through a multiquantum well superlattice”, Applied Physics Letters, 48, 478-480, 1986. 1994: First realization of QCL in InGaAs/AlInAs/InP pulsed operation, cryogenic conditions (Bell Labs) J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science, vol. 264, pp. 553–556, 1994.

Basic principles – Unipolarity Initial and final states have the same curvature the joint density of state is very sharp and typical of atomic transitions Laser emission from E3-E2 transition (photons) Phonon emission from E2-E1 transition (crystal vibrations) E2-E1 transition is fast: it is made resonant with the optical phonon energy Emission of photons occurs at the same wavelength, thus provides large gain Gain is limited by the population inversion

Basic principles – Cascaded geometry Electron re-cycling due to cascaded structure: Each injected electron generates N photons (N is the number of stages) Potential to decrease the population inversion in each stage Reduced electron-electron scattering and thus of distribution broadening

Basic principles – Practical structure

Engineering issues Steps towards a QCL Quantum design of optical transitions Band structure Engineering Building blocks Single QW Coupled QWs Superlattice Engineering band structure and optical transitions Because of quantum confinement, the spacing between the subbands depends on the width of the well, and increases as the well size is decreased. This way, the emission wavelength depends on the layer thicknesses and not on the bandgap of the constituent materials. Electron lifetime engineering is necessary to fulfill the population inversion condition: τ32 > τ21

Operation – Emission wavelengths Emission wavelength does not depend on the material system Development of lasers with different wavelengths using the same base semiconductors: - from 3.5 to 24 µm InGaAs and AlInAs grown on InP - far-infrared lasers based on the GaAs/AlGaAs material system Shortest emission wavelength: 2.9 μm from InAs/AlSb

QCL performance advantages The same semiconductor material can be used to manufacture lasers operating across the whole mid-infrared (and potentially even farther in the Far-Infrared) range. It is based on a cascade of identical stages (typically 20-50), allowing one electron to emit many photons, emitting more optical power. It is intrisically more robust (no interface recombination). Since the dominant non-radiative recombination mechanism is optical phonon emission and not Auger effect (as it is the case in narrow-gap materials), it allows intrinsically higher operating temperature. As of now, it is still the only mid-infrared semiconductor laser operating at and above room temperature. Potential for very high speed modulation: - absence of relaxation oscillations due to fast non-radiative relaxation rates - bandwidth determined by the photon lifetime in the cavity, - hence no advantage, rates up to 10 GHz Delta-like joint density of states: - symmetric gain curve - zero refractive index change at the gain peak - low alpha (LEF) parameter - no frequency modulation with direct modulation - low linewidth

QCL performance highlights Wavelength agility - 3.5 to 24 μm (AlInAs/GaInAs), 60 to 160 μm (AlGaAs/GaAs) - Multi-wavelength and ultrabroadband operation High optical power at room temperature: > 1 W pulsed, 0.6 W cw Narrow linewidth: < 100 kHz; stabilized < 10 kHz Ultra-fast operation: - Gain switching (50 ps) - Modelocking (3-5 ps) Applications: trace gas analysis, combustion & medical diagnostics, environmental monitoring, military and law enforcement Reliability, reproducibility, long-term stability Industrial Research and Commercialization: Hamamatsu, Thales, Pranalytica, Alpes Lasers, Maxion, Laser Components, Nanoplus, Cascade Technologies, Q-MACS Fraunhofer Institute, PSI, Aerodyne

QCL challenges Room temperature cw operation very high threshold power densities that generate strong self-heating of the devices Tunable over a broader range Development of QCL at telecom wavelengths Increase output power Mode locking of QCLs for sub-ps generation QCLs based on valence-band intersubband transitions in SiGe/Si quantum wells Challenges within CLARITY project - low noise QCLs - sub-shot noise generation - proposed solution: injection locking

QCL noise-reduction with injection locking Investigation of low noise operation using injection locking (IL) Slave laser locks on the injected master laser Noise performance is evaluated by the Relative Intensity Noise (RIN) Strong suppression of the slave laser RIN spectrum is expected Actual RIN reduction should be identified by correlation with the emitted power Within CLARITY alternative IL techniques are used in order to approach sub-shot noise operation